Lean burn engines usually operate at an air/fuel ratio (A/F) .gtoreq. 18 to obtain improved fuel economy. However, the usual three-way catalyst (TWC) is most efficient at approximately stoichiometry, (A/F=14.65). Accordingly, it has been proposed to locate a NO.sub.x trap downstream of the TWC to store NO.sub.x during lean A/F operation and subsequently converting the NO.sub.x to N.sub.2 and O.sub.2 by operating the engine at a rich A/F. This NO.sub.x conversion occurs within an optimum temperature window of approximately 300.degree. C. to 400.degree. C. The trap is preferably located underbody so that during hard, wide-open throttle (WOT) driving, the trap temperature does not exceed 800.degree. C. The temperature of the three-way catalyst should not exceed approximately 1000.degree. C.
When operating with fuel containing contaminants such as sulfur, an accumulation of the contaminants in the trap causes a decrease in the amount of NO.sub.x the trap can absorb. The contaminants must be "burned" off or desorbed at temperatures .gtoreq. 675.degree. C. and with an A/F .ltoreq. 14.65 (rich). A previous method of raising the temperature of the trap from its usual operating temperature of approximately 300.degree. to 400.degree. C. to a temperature of at least 675.degree. C., employs secondary air injection. This approach to SO.sub.x purging or desulfation of contaminants is expensive. Another approach to the problem employs a split exhaust manifold, separated TWC, or separated exhaust pipes, as proposed in copending application Ser. No. 764,185 filed Dec. 13, 1996 and assigned to the assignee of the present invention.